Mutual information signal communication between two communication terminals, such as portable telephones, personal computers or the like, is usually carried out through a predetermined communication network in the form of wireless system or wire system. There has been proposed a relatively new style of the communication network by the name of mesh network.
The mesh network is different from known communication networks each constituted with a base node which functions as a central controller and a plurality of relay nodes, each of which operates under the control by the base node. In the mesh network, any node corresponding to the base node functioning as the central controller is not provided but a plurality of relay nodes are provided in such a manner that each contiguous two of the relay nodes are placed with their communication areas overlapping partially with each other so that communication paths extend in a reticulated pattern. With the mesh network, the following advantages can be obtained. For example, when a certain one of the relay nodes provided in the mesh network is put in an obstructive condition for interrupting communication temporarily, the communication is rapidly revived through another relay node provided in the mesh network. In addition, it is easy to add a new relay node to the network.
The mesh network constituted with a plurality of relay nodes wherein communication between a communication terminal and the relay node or between the relay nodes is carried out by means of wireless is named a so called wireless mesh network. One of serious problems arising in the wireless mesh network is a trouble brought about by electric wave collision. In such a trouble, electric waves transmitted respectively from a plurality of transmission sources come into collision with each other on a certain communication channel so as to obstruct propagation of the electric waves and therefore the communication through the same communication channel cannot be appropriately performed.
Accordingly, various methods or measures have been proposed for avoiding the electric wave collision arising on the communication channel as described above. One of such methods or measures is, for example, the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) system (as disclosed in, for example, non patent document 1). In the CSMA/CA system, when an information signal is intended to be transmitted, first, the condition of occupation on the communication channel selected to transmit the information signal is detected. (The detection carried out in such a manner is called “carrier sense”.) Then, if it is clarified that the communication channel is in an idle state wherein the communication channel is not used for communication, as a result of the carrier sense, the information signal transmission is commenced when the idle state continues for a predetermined period of time. On the other hand, if it is clarified that the communication channel is in a busy state wherein the communication channel is used for communication, as a result of the carrier sense, the commencement of the information signal transmission is delayed until the communication channel comes to the idle state. Consequently, the electric wave collision on the communication channel selected to transmit the information signal is restricted.
Another of the methods or measures proposed for avoiding the electric wave collision arising on the communication channel is, for example, the RTS-CTS (Request To Send-Clear To Send) system (as disclosed in, for example, non patent document 2). In the RTS-CTS system, when an information signal is intended to be transmitted, first, the relay node in the transmission side is operative to transmit RTS information which represents the intention of the information signal transmission. The relay node in the reception side which has correctly received the RTS information is operative to transmit CTS information which represents the completion in preparations for receiving the information signal. Then, the relay node in the transmission side which has correctly received the CTS information is further operative to commence the information signal transmission. Consequently, the electric wave collision on a communication channel selected to transmit the information signal is restricted. There is also proposed the MARCH (Media Access with Reduced Handshake) system in which interception of wireless signals is utilized so that only a transmission source is required to transmit the RTS information and each of the relay nodes is not required to transmit the RTS information but required to transmit the CTS information, as a kind of the RTS-CTS system different from the standard RTS-CTS system in which each of the relay nodes is operative to transmit and receive the RTS information and the CTS information. With the MARCH system, when the information signal is sequentially relayed by a plurality of relay nodes, the number of operations for transmitting and receiving the RTS information and the CTS information carried out in each of the relay node, each of which is called “handshake”, can be reduced.
The non patent document 1: Syozoh Komaki, et al., “Wireless Technology and Applications Thereof 3: Wireless LAN and Ubiquitous Network”, Maruzen Kabushikigaisha, pp. 109 to 118, 30 Jan. 2004
The non patent document 2: C-K. Toh, “Ad Hoc Mobile Wireless Networks”, First Edition, Kozo Keikaku Engineering Inc., pp. 44 to 53, 31 May 2003